TITOLO: Electron diffractive imaging of oxygen atoms in nanocrystals at sub-ångström resolution
RIVISTA: Nature Nanotechnology 2010, 5 (5), 360-365
AUTORI: Liberato DE CARO ((e-mail: liberato.decaro@ic.cnr.it)
Elvio CARLINO ((e-mail: carlino@tasc.infm.it)
Gianvito CAPUTO (e-mail: gianvito.caputo@unisalento.it)
P. Davide COZZOLI (e-mail davide.cozzoli@unisalento.it)
Cinzia GIANNINI ((e-mail: cinzia.gianninu@ic.cnr.it)
ABSTRACT
High-resolution imaging of low-atomic-number chemical elements using electron microscopy is
challenging and may require the use of high doses of electrons. Electron diffractive imaging, which
creates real-space images using diffraction intensities and phase retrieval methods, could overcome
such issues, although it is also subject to limitations. Here, we show that a combination of electron
diffractive imaging and high-resolution transmission electron microscopy can image individual
TiO2 nanocrystals with a resolution of 70 pm while exposing the specimen to a low dose of
electrons. Our approach, which does not require spherical and chromatic aberration correction, can
reveal the location of light atoms (oxygen) in the crystal lattice. We find that the unit cell in
nanoscale TiO2 is subtly different to that in the corresponding bulk.
TESTO
High-Resolution Transmission Electron Microscopy (HRTEM) has revolutionized our
understanding of nanoscale materials by identifying structure-properties correlations at the atomic
level. The spatial resolution achievable by TEM is related to the short wavelength of the highenergy electrons (e.g. 2.5 pm for 200 keV) used for imaging samples. However, despite technical
progress achieved in the construction of modern microscopes the diffraction limit has not yet been
approached due to the aberrations of electromagnetic lenses. In 2010 a joint efforts made by
researchers of the Istituto di Cristallografia, Istituto Officina dei Materiali and National
Nanotechnology Laboratory at the Istituto Nanoscienze of CNR have demonstrated that the imaging
resolution of a HRTEM experiment can be improved by using an approach that bypasses such
drawbacks. It relied on recording a phase-contrast HRTEM image of the target object together with
the corresponding electron nano-diffraction (n-ED) pattern with a standard HRTEM microscope.
While the resolution of the HRTEM image was limited to 0.19 nm, the n-ED pattern contained
reflections corresponding to significantly smaller lattice spacings. The resolution of the n-ED
pattern was hence much higher than that of the HRTEM image. A new phase retrieval algorithm
was developed to extract the information contained in the n-ED pattern by using the information
contained in the HRTEM image as input data. This approach is called electron diffractive imaging
(EDI). Figure 1 shows an individual crystalline TiO2 (anatase) nanorod that was imaged by EDI at a
record resolution of 70 pm, unambiguously revealing the presence and location of light atomic
elements, namely oxygen, in the relevant tetragonal lattice. Subtle deformation of the anatase unit
cell was also detected, which can be responsible for some of the peculiar chemical-physical
properties of these nanostructured TiO2 materials. With EDI the study and understanding of matter
at ultimate resolution is now possible using standard HRTEM microscopes.
Figure 1 (a) HRTEM image of an anatase TiO2 nanorod down the [100] zone axis;( b) Combination
of the fast Fourier transform (FFT) of (a) with the n-ED pattern of the relevant nanorod after
subtraction of the contribution of the amorphous carbon substrate; (c) Magnified view of the
HRTEM image contrast in (a); (d) EDI-retrieved image, where the rectangular box highlights the
TiO2 lattice along the [100] direction (blue: O atoms, red: Ti atoms).